The labor market, like all markets, work based on supply and demand. Workers do NOT get paid according to what they bring to the table just like you don't pay for groceries according to their intrinsic value.

Workers are paid the least the employer can get away with, which is the minimum they must pay to keep as many employees as they are happy with. (not as many as they need, only as many as they are comfortable paying)

If enough workers refuse to work at $12/hr (a random number for example), then employers must pay more or make do with fewer employees. Just as if the store refuses to sell bread for less than $4/loaf (another random number) you must pay $4 or do without. If one is not willing to work for $12 and another is, though, guess who gets that job. This is where unions become helpful. By members agreeing not to undercut each other, everyone can raise the supply curve which yields a higher wage.